While DCC is primarily meant to power and communicate with the trains on the tracks, there are circumstances where having some auxiliary power available would be nice without having to run an extra set of wires. Maybe powering a remote turnout, an IR sensor, some animation or building lighting, or a fast clock secondary display?
Fortunately, harvesting power from the DCC bus is easy. The DCC waveform is an AC square wave. This can be converted to DC with the help of a bridge rectifier, which is simply 4 diodes arranged so that, no matter which polarity is at the input terminals, the same polarity voltage appears at the output. After rectifying the DCC waveform, the output voltage should be filtered with a capacitor. This results in relatively clean DC power at the output.
While simple, the above circuit does have some limitations. First, the output is unregulated. This means that whatever it is powering must withstand the output voltage, which in turn is derived from the DCC bus voltage. While the DCC bus voltage is typically 12-16V, some command stations can put out as much as 22V. Just be careful and check what your command station applies to the track before connecting any sensitive electronics to the output.
Secondly, whatever is powered by the above circuit will lose power if the command station ever shuts down or a track circuit breaker opens, maybe due to a short across the tracks. Depending on what is being powered, this can cause the load to lose its state – you might not want to power an entire signal system this way, for example. You’ll also be using up part of your booster’s current capacity to power accessories rather than trains, so plan accordingly.
Given the above statements, you might be wondering why we mentioned powering a fast clock secondary display this way. That’s a good question. We don’t recommend powering the master clock from the track bus for exactly the reasons above – if the DCC power goes away, you lose the time. However, the secondary display simply (and automatically) syncs with the master. This means, once track power is re-established, the secondary displays will automatically start displaying the time again – or Hold mode if that was activated. Therefore, extracting power from the DCC bus may be a convenient and low cost solution to powering the secondary displays, especially when using a wireless fast clock system.
How do I Get One?
The circuit is easy and inexpensive to build. Almost any bridge rectifier can be used as long as it is rated for at least the expected load current. The capacitor is not very critical – just make sure it is rated for at least 2x the expected output voltage and is around 100uF or larger. Soldering the circuit to a piece of protoboard is a relatively simple exercise. Some parts that work well for this application are:
If you don’t want to build your own from scratch, kits (and fully assembled versions) are also available, complete with terminal blocks on both the input and output.